Device and process for switching and controlling an electron dose emitted by a micro-emitter

ABSTRACT

This invention relates to a device for switching and controlling an electron dose emitted by a micro-emitter comprising  
     a sensor module ( 30 ) that receives the output current from the micro-emitter and a voltage to adjust the polarization point of the said device,  
     a comparator module that receives a threshold voltage to adjust the quantity of charges to be emitted,  
     a logical module to initialize the electron emission, and to define whether or not the micro-emitter should emit,  
     a control module that generates the voltages necessary for initialization and extinction of the micro-emitter current pulse,  
     means of varying the threshold voltage.

TECHNICAL FIELD

[0001] This invention relates to a device and process for switching andcontrolling an electron dose emitted by a micro-emitter, for example amicrotip.

STATE OF PRIOR ART

[0002] Microtip type micro-emitters will be considered in the remainderof the description, as a non-restrictive example.

[0003] The subject of microtips, now accompanied by the subject ofnanotubes, defines a range of applications for FED (Field EmissionDisplay) displays and also for micro-emitters, in which requirements interms of switching and controlling of emitted flows are very severe.

[0004] In the case of a hot emission (diodes, triodes, cathode raytubes), electrons acquire sufficient energy (called “output work”) dueto their thermal agitation to go beyond the potential barrier thatretains them to nuclei. They are then moved towards the material surfaceand, if there is an electric field that attracts them, they can beextracted from this material. At ordinary temperatures, the thermalagitation energy is not sufficient for electrons to exit from thematerial.

[0005] In the case of a cold emission, based on the principle of a fieldeffect test in a vacuum chamber, a tunnel effect enables electrons to beextracted from the emitter (cathode) in the vacuum and then to becollected on an anode. Emitters working in cold emission are consideredas being voltage controlled current sources, the flow of emittedelectrons obeying Fowler-Nordheim equations.

[0006] For example, this is the case of a microtip 10 made of Tungstenused as an electron emitter. Its electrical scheme is shown in FIG. 1A.An electron flow is set up between the anode 11 and the cathode 12. Acontrol voltage is applied between the extraction grid 13 called the“gate”, and the cathode 12. FIG. 1B shows the behavioral symbol of sucha microtip 10 that can be used with a generic electrical simulator(“Spice” type)

[0007] The emission condition for such a microtip 10 is characterized bystrong non-linearity of the emission current I_(tip) as a function ofthe voltage applied on the extraction grid 13. The tip current I_(tip)satisfies the law:

I _(tip) =a _(fn) V ²exp^(−b) ^(_(fn)) /V _(gc)

[0008] The coefficients a_(fn) and b_(fn) depend on the geometriccharacteristics of the microtip. One such current-voltage characteristicis illustrated in FIG. 2. An example of an operating point(I_(tip)=I_(on) for V_(gate−cathode)=V_(on)) is shown in this figure.The ideal characteristic is shown as reference 14.

[0009] In reality, this type of characteristic cannot be reproduced fromone microtip to another. The result is curves 15 shown in dashed lines.

[0010] Therefore one of the disadvantages of cold emission is to revealsome instability in the value of the current, which is equivalent to anoise generated by output working fluctuations inherent to local surfacecontaminations. These fluctuations are variable from one microtip toanother and are also variable in time for the same microtip.

[0011] There are two possible types of microtip control:

[0012] a current control by a current regulation device: this type ofpossibility is used in FEDs (Field Emission Display) through a single or“multigate” transistor located in series in the cathode circuit, asdescribed in document references [1] and [2] at the end of thedescription. The current emitted by each microtip may be programmedtheoretically. It is independent of the quality and characteristics ofeach microtip. The voltage V_(gc) is modulated from one microtip toanother or in time. One of the defects of such a control is that itmixes a low voltage (LV) and a high voltage (HV) at the transistorswitching and controlling circuit, because the extraction electrode mustbe increased to a few tens of Volts. The visual display matches thelimited operating precision frequency of this type of control.

[0013] a voltage control: if care is not taken, the emission currentwill be modulated which may be unacceptable for some applications. Ifthe current excursion and particularly the extreme values are known, andif the quantity to be controlled is the electric charge, this type ofsolution is satisfactory when it is combined with a variable observationtime window, T_(nom).$Q = {{I_{nom}*T_{nom}} = {{2I_{nom}*\frac{T_{nom}}{2}} = {\frac{I_{nom}}{2}*2T_{nom}}}}$

[0014] The device according to the invention is a circuit of this typethat is naturally faster and in which the observed linearity defects arecorrected, the extraction grid HV control circuits being independent ofthe LV circuits controlling the electric charge, which simplifies use ofthe circuit and reduces the sensitivity to noise.

[0015] Therefore several solutions are possible to measure the quantityof electrons transmitted by a microtip. In some cases, it is possible towork in current regulation as illustrated in FIGS. 3A and 3B. Emissionof a calibrated current (generator 16) for a given time delimits anelectric charge according to the law Q=T.t. This type of currentregulation system includes a sensitive tip current detection element 17,a reference current test element 18 and a current adjustment element 19.This system may operate:

[0016] In open loop in the case of a sequential calibration, thenprogramming of a given number of measurements with a single reference,as illustrated in FIG. 3A,

[0017] In closed loop in the case of a current servocontrol in real timeas illustrated in FIG. 3B, and as described in document reference [3].

[0018] In the embodiment illustrated in FIG. 3A, the specification forthe system must allow the necessary time to perform the calibrations.This type of implementation is incapable of correcting imperfections inthe electron beam for which the recurrence frequency is higher than thecalibration refreshment frequency.

[0019] In the embodiment shown in FIG. 3B, the stability of thecounter-reaction loop is essential and it must be guaranteed, usually atthe price of active compensation of the pass band of the looped systemand therefore to the detriment of its speed performances.

[0020] Requirements in terms of speed, stability, noise and linearitymake it impossible to use this type of implementation in manyapplications.

[0021] A global method of controlling weak electric charges consists ofdefining the required quantity of charges, interrupting the electronbeam when the required dose has been reached (“dose control”), usingseveral configuration input variables. In this case, the quantity ofelectric charges is defined in advance. The device used for this controlmust operate on a tip current dynamic, particularly including currentfluctuations in time for the same microtip. Theoretically, this type ofmethod enables very good linearity. However, the use of real functionalmodules and the requirement for high frequency operation result instrong non-linearities in the electric charge controlled as a functionof the current state.

[0022] A document reference [4] according to known art mentioned at theend of the description describes a two-dimensional network of miniaturecathodes used as electron beam emitters that are numericallyaddressable. This network includes internal electron focusing for eachemitter, a closed loop electron dose control circuit for controllingeach emitter by precisely controlling the electron flow. This type ofdose control circuit connected to an emitter can be used to obtain adose delivered during each cycle write, adapted despiteemitter-to-emitter mismatch, temperature and ageing effects. Thiscontrol circuit terminates the emission at a fixed dose rather than at afixed time. It is an integrated component and is connected to theemitter.

[0023] But this type of control circuit is a source of non-linearities.Furthermore, for a linear or two-dimensional arrangement of microtips,it cannot compensate for dispersions of doses emitted due to currentdispersions inherent to microtips.

[0024] The purpose of the invention is to compensate for this type ofnon-linearity so as to make the control device linear and useable, andto provide specific solutions for linear or two-dimensional devices.

PRESENTATION OF THE INVENTION

[0025] The invention relates to a switching and controlling device foran electron dose emitted by a micro-emitter, for example a microtip,characterized in that it comprises:

[0026] a sensor module that receives the output current from themicro-emitter and a voltage to adjust the polarization point of the saiddevice,

[0027] a comparator module that receives the output signal from the saidsensor module, and a threshold voltage to adjust the quantity ofelectrons to be emitted,

[0028] a logical module that receives the output signal from thecomparator module, and a start signal to initialize the electronemission, and a logical signal to define whether or not themicro-emitter should emit,

[0029] a control module that receives the output signal from the saidlogical module that generates the voltages necessary for initializationand extinction of the micro-emitter current pulse,

[0030] means of varying the threshold voltage such that the sumS=N_(start)+N_(measure)+N_(off) remains constant during the electronemission, where N_(start) is the number of electrons at the currentpulse start time, N_(measure) is the number of electrons at themeasurement time of this current pulse, N_(off) is the number ofelectrons at the extinguishing time of this current pulse.

[0031] In a first example embodiment, the device according to theinvention comprises means of modulating the threshold voltage in timestarting from the initialization signal so as to program a variable dosecontrol in time such that excess electrons emitted during theinitialization and extinguishing times are strictly compensated by areduction of the programmed dose in time.

[0032] In a second embodiment, the device according to the inventionalso comprises:

[0033] a module for detecting the micro-emitter current, capable ofreproducing the tip current I_(tip) exactly, or adding a gain on thecurrent,

[0034] a variable voltage generation module that outputs a set voltageV2=f(I_(tip)).

[0035] The invention also relates to a linear or matrix switching andcontrolling device for electron doses emitted by a set ofmicro-emitters, characterized in that it comprises the following foreach micro-emitter:

[0036] a sensor module that receives the output current from themicro-emitter and a voltage to adjust the polarization point,

[0037] a comparator module that receives the output signal from the saidsensor module and a threshold voltage to adjust the quantity ofelectrons to be emitted,

[0038] a logical module that receives the output signal from thecomparator module, and a start signal to initialize the electronemission, and a logical signal to define whether or not themicro-emitter should emit,

[0039] a control module that receives the output signal from the saidlogical module that generates the voltages necessary for initializationand extinction of the micro-emitter current pulse,

[0040] means of varying the threshold voltage such that during theelectron emission, the sum S=N_(start)+N_(measure)+N_(off) remainsapproximately constant, where N_(start) is the number of electrons atthe current pulse start time, N_(measure) is the number of electrons atthe measurement time of this current pulse, N_(off) is the number ofelectrons at the extinguishing time of this current pulse.

[0041] The invention also relates to a process for switching andcontrolling an electron dose emitted by a micro-emitter comprising:

[0042] a step to convert the current output by the micro-emitter and toadjust the operating polarization point,

[0043] a step to compare the signal obtained at the output from theprevious step with a threshold voltage for adjustment of the electronquantity to be emitted,

[0044] a logical step to initialize the electron emission, and to definewhether or not the micro-emitter should emit,

[0045] a control step that generates the voltages necessary forinitialization and for extinction of the micro-emitter current pulse,

[0046] characterized in that it comprises a step to vary the thresholdvoltage such that during the emission of electrons, the sumS=N_(start)+N_(measure)+N_(off) remains constant, N_(start) being thenumber of electrons at the current pulse start time, N_(measure) beingthe number of electrons at the measurement time of this current pulse,N_(off) being the number of electrons at the extinguishing time of thiscurrent pulse.

[0047] This type of invention has a wide field of applications:

[0048] electron emission by cold cathode,

[0049] switching and controlling of weak electric charges,

[0050] compensation of charge measurement errors,

[0051] high operating frequency,

[0052] solution compatible with application specific integrated circuits(ASICs).

BRIEF DESCRIPTION OF THE DRAWINGS

[0053]FIGS. 1A and 1B illustrate the electrical scheme and thebehavioral symbol of a microtip, respectively,

[0054]FIG. 2 illustrates the current-voltage characteristics of amicrotip,

[0055]FIGS. 3A and 3B illustrate a current regulation system for an openloop microtip and a closed loop microtip, respectively,

[0056]FIG. 4 illustrates a switching and controlling device for anelectron dose emitted by a microtip,

[0057]FIG. 5 illustrates the sensor module of the device in FIG. 4,

[0058]FIG. 6 illustrates the comparator module of the device in FIG. 4,

[0059]FIGS. 7A and 7B show time diagrams illustrating operation of thedevice in FIG. 4,

[0060]FIG. 8 illustrates a materialization of the error on the number ofprogrammed electrons,

[0061]FIG. 9 illustrates error compensation on the number of electronsprogrammed by variable threshold,

[0062]FIG. 10 illustrates the decomposition of a current pulse inelementary times,

[0063]FIG. 11 illustrates a decomposition that is simpler than thatillustrated in FIG. 10,

[0064]FIG. 12 illustrates the distribution of doses during the differentelementary times,

[0065]FIGS. 13A and 13B illustrate curves giving the number of electronsrelative to the tip current, without using compensation and using activecompensation on the current, respectively,

[0066]FIG. 14 illustrates an example time compensation according to theinvention,

[0067]FIGS. 15 and 16 illustrate a simplified compensation scheme as afunction of the tip current according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

[0068] The switching and controlling device of an electron dose emittedby a micro-emitter illustrated in FIG. 4 is composed of a microtip 10with an anode 11, a cathode 12 and an extraction grid 13, capable ofsupplying a current when the voltage of the extraction grid 13 relativeto the cathode 12 becomes greater than the extraction voltage in thevacuum. Parasite capacitances 20 and 21 are inherent to the fabricationof such a microtip 10 in microtechnology.

[0069] This device comprises:

[0070] a sensor module 30 that performs an electron-voltage conversionand that receives the current Ic output by this microtip 10 and avoltage V1 to adjust the polarization point of the said device, thesensitivity of a module

being expressed in Volts/electrons,

[0071] a comparator module 31 that receives the output signal Vse fromthe said sensor module 30 and a threshold voltage V2 to adjust thequantity of electrons to be emitted, and that outputs a sufficientcharge detection signal Vcom,

[0072] a logical module 32 that receives this signal Vcom, and a Startsignal to initialize the electron emission, and a logical data signal todefine whether or not the microtip should emit,

[0073] a control module 33 that receives the output signal from the saidlogical module 32 and the Vg-on and Vg-off signals that generate thevoltages necessary for initialization and extinction of the microtipcurrent pulse (several tens of Volts).

[0074] This device is actually applicable to an arrangement of severalmicrotips either in the form of a linear arrangement (strip) or atwo-dimensional arrangement (matrix). All combinations of arrangementsare also possible. This device can be made using a specific high voltagetechnology, and can control electron doses emitted at high rates.

[0075] We will now analyze each of these modules 30, 31, 32 and 33.

Sensor Module 30

[0076] The role of this module 30 is to process the basic informationavailable on the microtip 10 and to convert it into a magnitude that canbe compared with an input magnitude, in order to take a decision on thenumber N of electrons emitted.

[0077] This module may advantageously be composed of a CTIA (CapacitiveTransImpedance Amplifier) amplifier that makes a current—voltageconversion. The input variable is then the cathode current of themicrotip I_(c). This amplifier is characterized by its conversion gain

that is expressed in Volts/e⁻. It is composed of an amplifier 35, acounter reaction capacitor (C_(fb)) 36 and a reset device 37. The resultfor the output excursion ΔV_(s) of the sensor module is:${\Delta \quad V_{s}} = {\frac{{- I_{c}}*T_{i\quad n\quad t}}{C_{f\quad b}} = {\frac{N*q_{e}}{C_{f\quad b}} = {N\quad }}}$

[0078] This type of solution is advantageous compared with a solutionmaking a direct integration on the microtip capacitance for severalreasons:

[0079] the signal is not sensitive to parasite capacitances on the inputside,

[0080] its conversion gain may be fixed precisely. It is defined by thevalue of C_(fb). For example, it may be 23 μV/e⁻ for C_(fb)=7 fF,

[0081] the cathode polarization point is fixed by the external variableV1.

Comparator Module 31

[0082] This module 31 receives two analog voltages on its inputs:

[0083] the output voltage V_(sc) from the sensor module 30,

[0084] the control voltage V2 that fixes the value of the comparisonthreshold.

[0085] This module comprises an amplifier 40 in open loop, for which theoutput level comprises two states (VDD and VSS) equivalent to twological states as a function of the input voltages:

[0086] as long as V_(sc)>V2, the logical output V_(com) remains equal to“1”,

[0087] when V_(sc)=V2, the logical output V_(com) switches and is set toa logical “0”.

Logical Module 32

[0088] This module 32 has several internal signal sequencing andgeneration functions. Its roles are to:

[0089] latch the decision made V_(com) obtained at the output from thecomparator module 31 until the arrival of a reset signal,

[0090] generate non-overlapping phases useful for resetting the sensormodule 30 and the control module 33.

[0091] This module is initialized by a start signal at the beginning ofthe sequence, and obeys the data signal as illustrated in the followingtable: Data Action 1 Emission from the microtip 0 No emission from themicrotip

Control Module 33

[0092] This module 33 establishes the extraction grid voltage necessaryfor the microtip to emit the required current synchronously with theappearance of the start signal. When the emitted electron dose has beenreached (decision signal Vcom emitted by the comparator module 31), thismodule 33 cuts off the flow by bringing the extraction grid voltage to alevel such that the electron current is reduced by several decades.These ignition and extinguishing values depend on the transconductanceof the microtip and its geometric model. Control voltages may beswitched from 20 V to about 50 V, which then requires the use of aspecific high voltage technology (HVCMOS). The main function of thismodule 33 is therefore to translate the level [0-3 V] to [20 V-50 V].

[0093] This type of switching and controlling device has manylimitations, inherent to the principle used. The voltage V_(se) obtainedat the output from the sensor module 30 is proportional to the cathodecurrent I_(c) emitted by microtip. Considering V1 as being theinitialization voltage level, the number N_(e) of electrons emitted bythe microtip is such that:$N_{e} = {\frac{Q_{e}}{q} = {{\frac{\left( {V_{s\quad e} - {V1}} \right)}{}\quad {where}\quad } = \frac{q}{C_{f\quad b}}}}$

[0094] Qe is the electric charge emitted and q is the charge of theelectron.

[0095] Therefore, a calibrated charge Qc may be programmed by V2 withthe following relation:${N\quad c} = {\frac{Q_{c}}{q} = \frac{\left( {{V2} - {V1}} \right)}{}}$

[0096] The value of the comparison threshold V2 fixes the programmedelectric charge. If all modules were perfect, the sensor module 30 wouldimmediately transmit a representation V_(se) of the cathode currentI_(c), the comparator module 31 would not have any delay, and theextraction grid control would instantaneously activate making orbreaking the electron flow, according to the time diagram in FIG. 7A.Regardless of the level of the electron current, the emitted chargewould be identical, and as illustrated in FIG. 7B:

[0097] a nominal current IC_(nom) would be interrupted after a certaintime T_(nom),

[0098] a current 2*IC_(nom) would be interrupted after a time t_(nom)/2,

[0099] a nominal current 0.5*Ic_(nom) would be interrupted after a time2*t_(nom).

[0100] The areas shown in grey in each of the three cases are equal.

[0101] In reality, the global duration of the current pulse is notlinear as a function of the programmed current level. Due to theparasite capacitances 20 and 21 mentioned above, switching of theextraction grid 13 by several tens of Volts temporarily disturbs theinput of the sensor module 30 for which the polarization has to bemaintained to prevent saturation. This type of saturation would thenrequire a large time constant before a return to equilibrium and wouldnot enable operation at high frequency. During this time in whichpolarization of the sensor module 30 is maintained to creation of theelectron flow, electron charges are already emitted and need to becounted in the global balance of emitted charges, although they cannotbe measured since they depend on the current level that is not known inadvance. This type of phenomenon is a first source of non-linearities.

[0102] Another phenomenon occurs when the electron beam is extinguished,when V_(se) reaches V2. The comparator module 31 has a delay in makingthe decision, which is inherent to any electron module. During thisdelay, the microtip 10 continues to emit and therefore there is anadditional extinguishing charge that is added into the global balance ofthe emitted charges. FIG. 8 shows a materialization of the error on thenumber N of programmed electrons, and illustrates such a phenomenon. Ifthe number of electrons emitted is plotted as a function of time withrespect to the number of programmed electrons, with a constant delay, anerror is observed on the number of electrons emitted depending on thecurrent level. In this figure, curve 45 corresponds to 2*Iinom, curve 46corresponds to Iinom and curve 47 corresponds to Iinom/2, curve 48corresponds to the number of electrons emitted. Therefore, there is anovershoot on the charge emitted with respect to the programmed charge,which is a second source of non-linearities.

[0103] A first solution for compensating for such non-linearities uses acomparison threshold that varies as a function of time. To achieve this,all that is necessary is to send a stair-case 50 on the input V2 of thecomparator module 31 as illustrated in FIG. 9.

[0104] The purpose of the invention is to compensate for suchnon-linearities by proposing other compensation methods by controllingthe cathode current in I_(c) and by feedback on the value of thethreshold V2.

[0105] By analyzing the profile 55 of the microtip current pulse, it canbe decomposed into a series of elementary times t1 to t6:

[0106] t1: time to set up the voltage Vgate + reset CTIA,

[0107] t2: latching time for the CTIA reset to cancel charge injectioneffects and transients,

[0108] t3: measurement time,

[0109] t4: comparator decision making delay time,

[0110] t5: delay time due to cutoff of Vgate (logical),

[0111] t6: delay to stop the electron flow.

[0112] Some of these elementary times can be grouped together, to givethe following simplified model:

[0113] t1+t2=t_(start): initialization time that extends from t_(début)(corresponding to the beginning of the pulse) until t_(start-control)(corresponding to the effective beginning of the dose control),

[0114] t3=t_(measure): actually controllable measurement time thatextends from t_(start-control) until t_(start-control) (corresponding tothe end of the dose control),

[0115] t4+t5+t6=t_(off): extinguishing time that extends fromt_(end-control) until t_(fin) corresponding to the effective end of thedose emission.

[0116] If it is considered that the current reaches its nominal valueI_(steady-state) quickly during the initialization time t_(start) andthat it is latched for an extinguishing time t_(off), as a firstapproximation it is therefore constant during the entire duration of thecurrent pulse. At the beginning, the setup time for V_(gate) is shortand at the end, logical and extinguishing delays for V_(gate) arelargely dominated by the delay of the comparator module 31 when thedecision is being made.

[0117] The total dose emitted as a number of electrons can be expressedas follows:$N_{beam} = {N_{measure} + \frac{I_{{steady}\text{-}{state}}*\left( {t_{start} + t_{off}} \right)}{q_{e}}}$

[0118] where$N_{measure} = {{\frac{\left( {V_{2} - V_{1}} \right)}{}\quad {where}\quad } = {q_{e}/{Ctia}}}$

[0119] The predicted electron dose is fixed by N_(measure), but in factan excess dose can be added due to non-zero start and extinguishingtimes. FIG. 12 illustrates a curve showing the number of electronsemitted as a function of the current state.

[0120] In theory, as mentioned above, the number of electrons emittedshould remain the same regardless of the current I_(tip), as illustratedby the horizontal curve 56.

[0121] Curves 57 and 58 illustrate the number of electrons emittedduring the initialization and the extinguishing times respectively. Thesequencing may be such that the times t_(start) and t_(off) remainconstant regardless of the current, in other words the electrons emittedduring these times t_(start) and t_(off) only depend on the currentstate (affine function).

[0122] The number of emitted electrons appears on the curve 59 which,for any value of the abscissa X, represents the sum of the curves56+57+58.

[0123] The relative numeric indication obtained from these curves showsan error on the number of electrons emitted with respect to the setvalue by a factor of 1.3 to 2.6. This is not acceptable for the requiredemission control precision.

[0124] The purpose of the device according to the invention is to becapable of precisely emitting a programmed number of electronsregardless of the current state of the microtip and to interrupt theelectron beam as soon as this value has been reached. Therefore the sumof electrons emitted during each of the times described above mustremain constant, i.e., the total number of electrons emitted must belinear and constant regardless of the tip current I_(tip).

[0125] The law for variation of the number of electrons emitted duringthe initialization and extinguishing times for the current pulse (affinefunction) is known. Therefore, it is possible to act on the test of thenumber N_(measure) of electrons effectively measured such that the sumS=N_(start)+N_(measure)+N_(off) remains constant. In fact, N_(measure)therefore needs to decrease when I_(tip) increases.

[0126] To achieve this, the value of the threshold detection voltage V2is modified during the electron exposure. Compensation is made onexcessive electron quantities satisfying the following law:$\frac{I_{tip}*t}{q_{e}}$

[0127] Two types of compensation are possible: a time compensation or acompensation as a function of the current. FIGS. 13A and 13B illustratetheoretical curve 60 and measured curve 61 respectively, and theoreticalcurve 60 and measured curves 61′ of the relative number of electrons asa function of the tip current I_(tip) respectively, without compensationand with compensation respectively, as a function of the current. Curve61′ demonstrates the improvement to be obtained by using such an activecompensation as a function of the current.

[0128]FIG. 13B shows the stability of the number of electrons emitted asa function of the tip current, although there is an offset that remainsinherent to the method used. The time denoted t_(measure) cannot be zerosince in this case nothing would be tested. The minimum time necessaryfor the compensation to work correctly must be such that the noise addedby the sensor module 30 remains weak compared with the signal beingprocessed by this module (typically N_(offset)=400 electrons, or ΔV_(S)_(—) _(min)=8 mv).

[0129] The invention also relates to a linear or matrix switching andcontrolling device for electron doses emitted by a set ofmicro-emitters, that comprises different modules 30, 31, 32 and 33 andmeans of varying the threshold voltage as described above, for eachmicro-emitter.

EXAMPLE EMBODIMENTS

[0130] Time Compensation

[0131] This type of compensation is illustrated in FIG. 14. It does notcover all needs. It is capable of compensating for disparities betweenmicrotips, but not high frequency fluctuations on the same microtip.However, it can be used as soon as it is certain that the recurrencefrequency of current fluctuations is less than the frequency ofappearance of the programmed pulses. The threshold voltage V2 ismodulated in time starting from the start signal so as to program a dosecontrol variable in time such that excess electrons emitted during thet_(start) and t_(off) phases are precisely compensated by the reductionof the programmed dose with time.${{Programmed}\quad {dose}} = {N_{prog} = {{\frac{\left( {{{V2}(t)} - {V1}} \right)}{}\quad {where}{\quad \quad}} = \frac{q_{e}}{C_{tia}}}}$

[0132] This time variation is controlled by the generator 65.

[0133] Active Compensation as a Function of the Current

[0134] When the frequency of current fluctuations is such that thecurrent can vary for an elementary exposure time, the previous timecorrection is no longer sufficient. In the expression of the balance ofthe number of electrons emitted: $N_{e^{-}} = \frac{I_{tip}*T}{q_{e}}$

[0135] The two variables I_(tip) and T vary simultaneously during thetest. Therefore, it is no longer possible to test one of the variableswhile measuring the other. An active correction is necessary as afunction of the current.

[0136]FIG. 15 illustrates a simplified compensation diagram as afunction of the tip current. A tip current detection module 67 iscapable of precisely reproducing the tip current or introducing a gain(X) on this current, for example using a current mirror. This outputcurrent is measured by the sensor module 30. The input current I_(tip)is also used as a reference for the variable voltage generation module68 that outputs a set voltage V2=f(I_(tip)). The decision on the time isalways taken by the comparator module 31, but the decision threshold V2is indexed on the instantaneous value of the emission current. Theresult is thus optimum compensation.

[0137] More precisely, using the same notations as in FIG. 11, thenumber of electrons emitted in each of the phases can be calculated:

[0138] Initialization phase$N_{start} = {q\frac{I*\left( {t_{début\_ contrôle} - t_{début}} \right)}{q}}$

[0139] Measurement phase$N_{measure} = {q\quad \frac{I*\left( {t_{{fin\_ contr}\hat{o}{le}} - t_{d\overset{'}{e}{but\_ contr}\hat{o}{le}}} \right)}{q}}$

[0140] Extinction phase$N_{stop} = {q\quad \frac{I*\left( {t_{fin} - t_{{fin\_ contr}\hat{o}{le}}} \right)}{q}}$

[0141] The number of electrons deposited in excess to be compensated bymodifying the voltage V2 is equal to: $\begin{matrix}{{N_{start} + N_{stop}} = {\frac{I}{q}\left\lbrack {\left( {t_{d\overset{'}{e}{but\_ contr}\hat{o}{le}} - t_{d\overset{'}{e}{but}}} \right) + \left( {t_{fin} - t_{{fin\_ contr}\hat{o}{le}}} \right)} \right\rbrack}} \\{= {\frac{I}{q}\left\lbrack {t_{start} + t_{off}} \right\rbrack}}\end{matrix}$

[0142] Hence as a function of ΔV2: $\begin{matrix}{{N_{start} + N_{stop}} = \frac{C*\Delta \quad {V2}}{q}} & {namely} & {{\Delta \quad {V2}} = {\frac{I}{C}\left\lbrack {t_{start} + t_{off}} \right\rbrack}}\end{matrix}$

[0143] Since the capacitance of the sensor block and times└t_(start)+t_(off)┘ are known, the variation of V2 to be programmed isdirectly proportional to I. The voltage difference to be programmed withrespect to Vref (voltage to be applied to obtain the required doseduring the measurement phase if Nstart and Nstop did not exist), cantherefore be used, for example using a resistance R_(L) to set up avoltage R_(L)*I, where R_(L)=(t_(start)+t_(off))/C. In the special casein which the CTIA amplifier is recharged to a high state, this voltageR_(L)*I must be added to the voltage Vref to stop supply the microtipand therefore its emission, more quickly than in the ideal case (withoutNstart and Nstop).

[0144] For example, the block 68 in FIG. 15 can then be made asillustrated in FIG. 16.

[0145] The transistor dimensions are chosen to satisfy the specifiedfunction in a manner known by those skilled in the art.

[0146] This type of embodiment is advantageous in the sense that itenables carrying out all required functions close to or in the electronemission site, which has several advantages:

[0147] it individually compensates for non-uniformities in emission ofmicrotips or any other device,

[0148] it performs these various functions in an ASIC (ApplicationSpecific Integrated Circuit),

[0149] consequently, it participates in improving productionefficiencies of microtips and their life,

[0150] it is directly possible to access large two-dimensional emitterswithout making the various peripheral interfaces more complex (automaticprocessing of the in-pixel signal).

References

[0151] [1] “Structure optimisation of transistor-based Si field emitterarrays” by T. Matsukawa, K. Koge, S. Kanemaru, H. Tanoue and J. Itoh(TIDW'98, pages 671-674, FED 2-4)

[0152] [2] “Active matrix field-emitter arrays for the next-generationFEDs” by J. Itoh, S. Kanemaru, T. Matsukawa (199, SID)

[0153] [3] U.S. Pat. No. 6,392,355 B1

[0154] [4] “Digital electrostatic electron-beam array lithography” by L.R. Baylor, D. H. Lowndes, M. L. Simpson, C. E. Thomas, M. A. Guillorn,V. I. Merkulov, J. H. Whealton, E. D. Ellis, D. K. Hensley, A. V.Melechko (J.Vac.Sci.Technol. B20 (6), November-December 2002, pages2646-2650)

1. Device for switching and controlling an electron dose emitted by amicro-emitter, characterized in that it comprises: a sensor module (30)that receives the output current from the micro-emitter and a voltage toadjust the polarization point of the said device, a comparator module(31) that receives the output signal from the said sensor module, and athreshold voltage to adjust the quantity of electrons to be emitted, alogical module (32) that receives the output signal from the comparatormodule (31), and a start signal to initialize the electron emission, anda logical signal to define whether or not the micro-emitter should emit,a control module (33) that receives the output signal from the saidlogical module that generates the voltages necessary for initializationand extinction of the micro-emitter current pulse, means of varying thethreshold voltage such that the sum S=N_(start)+N_(measure)+N_(off)remains substantially constant during the electron emission, whereN_(start) is the number of electrons at the current pulse start time,N_(measure) is the number of electrons at the measurement time of thiscurrent pulse, and N_(off) is the number of electrons at theextinguishing time of this current pulse.
 2. Device according to claim1, comprising means of modulating the threshold voltage (V2) in timestarting from the initialization signal (start) so as to program anelectron dose control that is variable in time such that excesselectrons emitted during the start (t_(start)) and extinguishing(t_(off)) times are strictly compensated by a reduction of theprogrammed dose in time.
 3. Device according to either claim 1 or 2,comprising: a module for detecting the micro-emitter current (67),capable of reproducing the tip current I_(tip) exactly, or adding a gainon the current, a variable voltage generation module (68) that outputs aset voltage V2=f(I_(tip)).
 4. Linear or matrix switching and controllingdevice for electron doses emitted by a set of micro-emitters,characterized in that it comprises the following for each micro-emitter:a sensor module (30) that receives the output current from themicro-emitter and a voltage to adjust the polarization point, acomparator module (31) that receives the output signal from the saidsensor module and a threshold voltage to adjust the quantity ofelectrons to be emitted, a logical module (32) that receives the outputsignal from the comparator module (31), and a start signal to initializethe electron emission, and a logical signal to define whether or not themicro-emitter should emit, a control module (33) that receives theoutput signal from the said logical module that generates the voltagesnecessary for initialization and extinction of the micro-emitter currentpulse, means of varying the threshold voltage such that during theelectron emission, the sum S=N_(start)+N_(measure)+N_(off) remainssubstantially constant, where N_(start) is the number of electrons atthe current pulse start time, N_(measure) is the number of electrons atthe measurement time of this current pulse, N_(off) is the number ofelectrons at the extinguishing time of this current pulse.
 5. Deviceaccording to any one of the above claims, in which each micro-emitter isa microtip.
 6. Process for switching and controlling an electron doseemitted by a micro-emitter comprising: a step to convert the currentoutput by the micro-emitter and to adjust the operating polarizationpoint, a step to compare the signal obtained at the output from theprevious step with a threshold voltage for adjustment of the quantity ofelectrons to be emitted, a logical step to initialize the electronemission, and to define whether or not the micro-emitter should emit, acontrol step that generates the voltages necessary for initializationand for extinction of the micro-emitter current pulse, characterized inthat it comprises: a step to vary the threshold voltage (V2) such thatduring the emission of electrons, the sumS=N_(start)+N_(measure)+N_(off) remains approximately constant,N_(start) being the number of electrons at the current pulse start time,N_(measure) being the number of electrons at the measurement time ofthis current pulse, N_(off) being the number of electrons at theextinguishing time of this current pulse.
 7. Process according to claim6, comprising a step in which the threshold voltage (V2) is modulatedwith time starting from the initialization signal (start) so as toprogram an electron dose control that is variable in time such thatexcess electrons emitted during the start (t_(start)) and extinguishing(t_(off)) times are all or partly compensated by a reduction of theprogrammed dose in time
 8. Process according to claim 6, comprising: astep for detecting the tip current, capable of reproducing the tipcurrent I_(tip) exactly, or adding a gain on the current, a step togenerate a variable voltage (68) that outputs a set voltageV2=f(I_(tip)).